1. Field of the Invention
The invention relates to a semiconductor device, and more particularly to a semiconductor device having spacer with tapered profile.
2. Description of the Prior Art
In current semiconductor industry, polysilicon has been widely used as a gap-filling material for fabricating gate electrode of metal-oxide-semiconductor (MOS) transistors. However, the conventional polysilicon gate also faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of gate dielectric layer, reduces gate capacitance, and worsens driving force of the devices. In replacing polysilicon gates, work function metals have been developed to serve as a control electrode working in conjunction with high-K gate dielectric layers.
However, in current fabrication of high-k metal transistor, particularly during the stage when spacer is formed on the sidewall of gate structure, issues such as over-etching or undercut often arise and causing etching gas to etch through spacer until reaching the bottom of the gate structure. This induces erosion in high-k dielectric layer and/or bottom barrier metal (BBM) and affects the performance of the device substantially. Hence, how to resolve this issue has become an important task in this field.